This invention relates to a beam inspection apparatus, comprising a radiation source, a beam control system and a measuring space for positioning an object to be examined, and also comprising a detection device for the detection of electrons emerging from the object due to interaction between the beam and the object.
An inspection apparatus of this kind in the form of an electron beam apparatus for the inspection of chips is known from EP-A-196 958, and in the form of a scanning electron microscope from U.S. Pat. No. 4,438,332.
In the known apparatus an electron current originating from a measuring point, for example, a metal track on a chip, is measured. When this point is exposed to an electron beam, mainly secondary electrons emerge and, because the secondary emission coefficient is approximately 1 in appropriate circumstances which are assumed to exist here and hereinafter, the current intensity in the return beam is approximately equal to the current intensity of the primary beam. In the case of exposure to a light beam or more generally a photon beam, for example, an UV laser, mainly photoelectrons emerge. For the measurement of secondary electrons as well as photoelectrons, use is made of, for example, a luminescent element to which a photomultiplier tube is optically coupled. The persistence in the luminescent material and the spread in flight time for the low-energetic secondary electrons and photoelectrons make it desirable or even necessary to measure in a pulsed manner. To this end, known apparatus utilize a beam interruptor. Using a so-called boxcar circuit, the signal-to-noise ratio in the signal is improved by integration. The measurement by means of a boxcar circuit is described in detail in EP-A-196 958.
Because the dimensions of the elements to be inspected, for example, tracks in a chip, become increasingly smaller and at present already are in the order of magnitude of 0.5 .mu.m, measurements must utilize an electron spot or photon spot having a comparatively small cross-section in order to ensure adequate resolution. When an electron spot having a cross-section of, for example, 0.1 .mu.m is used, an electron beam current of at the most approximately 1 nA occurs for customary beam parameters and a secondary emission coefficient of approximately 1. When measurements are performed with a pulse duration of, for example, at the most approximately 0.5 ns, per pulse a charge corresponding to the charge of only a few electrons must be measured. In the case of exposure to a photon beam similar restrictions exist. Inter alia, because of a limited efficiency of the detector due to losses incurred in the interception of the electrons, it becomes increasingly more difficult to measure the secondary electron current for shorter pulses and/or smaller currents (spot diameter). In such cases the signal will readily become lost in the noise. Because the chips are increasingly more complex, an increasing number of measurements must be performed thereon for reliable inspection. Using a boxcar circuit, each time only one sample can be formed after each trigger. The use of a plurality of parallel connected boxcar circuits makes the inspection apparatus extremely complex.